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Demartus, king of Sparta from 515 until 491 BC of the Eurypontid line, successor to his father Ariston. As King he is known chiefly for his opposition to his colleague the other Spartan king Cleomenes I.
When Cleomenes attempted to make Isagoras tyrant in Athens, Demartus tried unsucessfully to frustrate his plans. In 401 BC Aegina was one of the states which gave the symbols of submission (earth and water) to Persia. Athens at once appealed to Sparta to punish this act of medism, and Cleomenes I, crossed over to the island, to arrest those who were responsible for it. His first attempt was unsuccessful due to interference from Demartus who did his utmost to bring Cleomenes into disfavour at home.
In relaliation Cleomenes urged Leotychides, a relative and personal enemy of Demaratus, to claim the throne on the ground that the latter was not really the son of Ariston but of Agetus, his mothers first husband. Cleomenes bribed the Delphic oracle, to pronounce in favor of Leotychides, who became king Leotychides II in 491 BC.
After the deposition of Demaratus, Cleomenes visited the island of Aegina for a second time, accompanied by his new colleague Leotychides, seized ten of the leading citizens and deposited them at Athens as hostages.
On his Abdication Demaratus was forced to flee. He went to the court of the Persian king Darius I, who gave him the cities of Pergamum, Teuthrania and Halisarna, where his descendants were still ruling at the beginning of the 4th century.
He accompanied Xerxes I on his invasion of Greece in 484 BC and is alledged to have warned Xerxes not to under estimate the Spartans before the Battle of Thermopylae: